Do you have a friend that has a Mac (older model with FireWire). If you do you maybe in luck here.
You'll need a firewire cable that connects your system to his. Then using the Startup keys you should be able to boot up your system in Target Disk Mode then connect your system like it was an external drive to your fiends system at which point the MacBook Pro will look like an external drive! Now you can first backup anything of importance to your fiends system or if you can get a USB thumb drive place the field on it for safe keeping. Here's the Apple T/N that goes over Target Disk Mode: Startup key combinations for Intel-based Macs.
Now the tough part: You'll need to start off running Disk Utility fixing any permission and disk issues (you'll likely need to run it though a few times) to make sure the disk is OK. See if that allowed your system to startup on its own.
If that didn't fix things, you may be facing a missing driver issue as the Gray installer disks sometimes didn't have all of the needed files (space issues on CD) for the older systems. You'll likely need to find a retail version of 10.6.8 (Snow Leopard) to fix your system. Using your friends system if it is running Snow Leopard you should be able to install it on to your system if the optical drive in your system is messed up.